The use of blow molding as a method for manufacturing various sorts of articles is generally well known. Typically, this process involves the use of a mold consisting of two separate halves or portions having cavities of particularly desired shapes and sizes. Usually, one extrudes a large-diameter, sealed tube of molten material (commonly referred to as a “parison” places the tube between the mold halves, and closes the mold around the tube. The mold halves are closed around the tube at what is referred to as the “parting line.” Fluid pressure is then introduced into the tube, forcing the molten tube against the walls of the cavities, conforming the tube to the shape thereof. The pressure is maintained until the molten material cools and solidifies. The pressure is then released, the mold halves are pulled apart, and the hardened article is ejected therefrom.
It is well known to form various protuberances on blow-molded parts. Examples include hinge receptacles, handles, tabs to accept mounting screws, and snap bosses for joining blow-molded components. Certain prior art forms these protuberances by pushing and compressing the molten material of the blow mold parison during the mold close portion of the molding cycle. The pushing and compressing is accomplished by the closing of the two mold halves. It is therefore always in a plane generally perpendicular to the parting line of the mold, although incidental lateral transfer sometimes occurs when a shaping wedge of angled leading surface is part of one or both halves of the mold.
In particular, the use of integrally formed, permanent interference snap bosses is well known as an assembly method to join molded or fabricated plastic components into a finished article. This technique eliminates the need for additional attachment pieces, thereby saving time and materials cost in the assembly process. It also avoids the need for adhesives, which increase costs and potentially introduce appearance problems through smearing or misapplication of the adhesive. Blow-molded double-wall components lend themselves to this attachment method in that they are somewhat resilient, which facilitates assembly, and yet rigid by reason of the structural qualities of the double wall.
One disadvantage of current methods for creating snap bosses and other protuberances is that the blow molding process tends to limit the designer's freedom in placing the protuberance. Generally speaking, snap bosses need to be rigid (have a heavy wall) and deep (to cause significant interference) to provide permanent attachment if the attached components are subjected to even moderately heavy usage. Other protuberances also must be rigid and deep in order to serve their purposes and withstand use. To meet both of these requirements it has been necessary to locate snap bosses and other protuberances at a parting line of the double wall article of which they are a part, in particular a parting line with flash adjacent to it. When the molten material is blown into a narrow cavity along a parting line to form a protuberance, excess air is able to escape at a parting line, allowing the material to enter the appendage and be blown tightly against the wall. However, attempts to locate protuberances away from a parting line by similar methods result in thin-walled, shallow protuberances, because without the air escape provided at a parting line, the material is unable to fully extend into narrow cavities, and is not blown tightly against the walls of the mold. Also, there is sometimes insufficient flash to create a sufficiently rigid protuberance.
There are occasions when it is either not desirable or not possible to work with a design incorporating a snap boss or other protuberance at a parting line. Further, there are other instances where the amount of material available from normal “layflat” in the snap boss or other protuberance area is insufficient to achieve the necessary rigidity and depth.
Certain proposed techniques provide a blow-molding apparatus with an inwardly projecting hollow boss away from a parting line by retracting a piston extended through the mold into the parison during the blowing process. One disadvantage of such techniques is that it is unclear how they could be used to make a protruding boss away from the parting line, as opposed to an inwardly projecting hollow boss. A second disadvantage is that the resulting inward projection has a thinner wall than the rest of the blow-molded apparatus, as these techniques essentially stretch the wall of the apparatus to a greater distance without adding additional material. When forming the protuberance, however, it is desired that it have a sufficiently thick wall, that is, be sufficiently rigid. As explained, this is particularly important when the protuberance is a snap boss intended to permanently affix two blow-molded components.
Other proposed techniques form protruding bosses with an injection method through the use of a piston. Material is injected into the mold, and a piston is compressed while the material is still molten in order to force extra material to form a boss with a greater density than the rest of the component. Although these techniques can successfully form strong protuberances, injecting material into molds results in solid components. Although the protuberance itself might be solid, the purpose of blow molding is to form hollow components, and therefore techniques suited for injection methods are not suited for blow molding. The problem discussed above with respect to blow molding, whereby material is unable to form a protuberance away from a parting line because there is no air escape, does not arise with solid components. Aside from the fact that molds for use with injection methods do not necessarily have parting lines, narrow cavities can simply be filled by injecting more material into the mold. Therefore, these techniques are not suited to creating blow-molded components with protuberances away from the parting line, where blowing additional material will not solve the problems caused by the lack of an air escape or lack of sufficient flash.
What is desired, therefore, is a method and apparatus that facilitates the creation of double-wall blow-molded components with one or more protuberances that are sufficiently rigid and deep, but not abutting a parting line of the mold.